The present invention relates to energetic materials, particularly to the manufacture of energetic materials, and more particularly to the manufacture of energetic materials using sol-gel chemistry.
Energetic materials are herein defined as any material which stores chemical energy in a fixed volume. Explosives, propellants, and pyrotechnics are examples of energetic materials. Reaction results from either shock or heat. Explosives and propellants may be thought of as a means of storing gas as a xe2x80x9csolidxe2x80x9d. Pyrotechnics typically release much of their energy as heat.
Energetic materials consist of fuels and oxidizers which are intimately mixed. This is done by incorporating fuels and oxidizers within one molecule or through chemical and physical mixtures of separate fuel and oxidizer ingredients. The material may also contain other constituents such as binders, plasticizers, stabilizers, pigments, etc.
Traditional manufacturing of energetic materials involves processing granular solids into parts. These materials may be pressed or cast to shape. Performance properties are strongly dependent on particle size distribution, surface area of the constituents, and void volume. In many cases achieving fast energy release rates, as well as insensitivity to unintended initiation, necessitates the use of small particles ( less than 100 xcexcm) which are intimately mixed. Reproducibility in performance is adversely affected by the difficulties of synthesizing and processing materials with the same particle morphology. Manufacturing these granular substances into complex shapes is often difficult due to limitations in processing highly solid filled materials.
An example of an existing limitation of processing granular solids is in manufacturing energetic materials for detonators. The state-of-the-art now requires the precise synthesis and recrystallization of explosive powders. These powders typically have high surface areas (e.g.,  greater than 1 m2/g). The powders are weighed and compacted at high pressures to make pellets. Handling fine grain powders is very difficult. Dimensional and mechanical tolerances may be very poor as the pellets may contain no binder. Changes in the density and dimensions of the pellets affect both initiation and detonation properties. Manufacturing rates are also low as the process is usually done one at a time. Certification of material is typically done by expensive, end-use detonation performance testing and not solely by chemical and physical characterization of the explosive powder. As these detonators or initiating explosives are sensitive, machining to shape pressed pellets is typically not done.
Another current limitation is producing precise intimate mixtures of fuels and oxidizers. The energy release rates of energetic materials are determined by the overall chemical reaction rate. Monomolecular energetic materials have the highest power as the energy release rates are primarily determined by intramolecular reactions. However, energy densities can be significantly higher in composite energetic materials. Reaction rates (power) in these systems are typically controlled by mass transport diffusion rates.
The present invention solves many of these prior problems by manufacturing energetic materials through the use of sol-gel chemistry. Sol-gel chemistry is known in the art and broadly described here, but described hereinafter in greater detail to provide an understanding of this technology. In one approach, using sol-gel chemistry, a solution including explosive materials is made. That solution is then gelled to form a cross-linked skeleton, which may be either inert, or reactive, or energetic, and a continuous liquid phase. The liquid phase is then extracted to produce either a xerogel or an aerogel. A solid so produced has high surface area and homogeneity. During the solution stage of the operation, the solution may be easily cast into molds to produce final parts. By applying a stress during removal of the liquid phase, dense parts may be obtained. Alternately, explosive molding powders may be made which can be used as feedstock for pressing operations. The molding beads will have a high degree of uniformity in the microstructure. As these beads are orders of magnitude larger than explosive powders used in traditional processing they may be easily handled. Also, through using sol-gel chemistry the intimacy of mixing can be dramatically improved over mixing granular solids. Numerous synthetic routes may be carried out utilizing sol-gel chemistry in the processing of energetic materials; these include solution addition, powder/particle addition, nano-composites, and functionalized solid networks.
It is an object of the present invention to produce energetic materials using sol-gel chemistry.
A further object of the invention is to minimize prior problems associated with the manufacturing of energetic materials by the use of sol-gel processing.
A further object of the invention is to produce explosives using sol-gel chemistry which enables microstructural control of the explosive material to the nanometer scale.
Another object of the invention is to manufacture energetic materials using sol-gel chemistry, whereby the intimacy of mixing can be controlled and dramatically improved over the prior procedures of mixing granular solids or epitaxial deposition.
Another object of the invention is produced by sol-gel processing a solid skeleton composed of fuel with the oxidizer trapped within the pores, or vice versa.
Another object of the invention is to provide a process using sol-gel chemistry to create a material that has high energy, such as a strategic rocket propellant with high power, such as an ideal explosive.
Another object of the invention is to provide a process utilizing sol-gel chemistry wherein the sensitivity of the energetic materials can be readily controlled.
Another object of the invention is to utilize sol-gel chemistry in the processing of energetic materials by at least solution addition, powder/particle addition, nano-composites, and functionalized solid networks.
Another object of the invention is to utilize sol-gel chemistry for the preparation of energetic materials with improved homogeneity, and/or can be cast to near-net shapes, and/or can be made into precision molding powders.
Other objects and advantages of the present invention will become apparent from the following description and the accompanying drawings. Basically, the invention involves sol-gel manufactured energetic material and a process utilizing sol-gel chemistry for producing energetic materials. Energetic materials manufactured using sol-gel chemistry possess superior properties in terms of microstructural homogeneity, and/or can be more easily processed and/or processed with greater precision and accuracy, than can be obtained by the prior known technology. The sol-gel manufactured energetic materials can be utilized as precision detonator explosives, precision explosives, propellants, pyrotechnics, and high power composites.
The sol-gel process is a synthetic chemical process where reactive monomers are mixed into a solution; polymerization occurs leading to a highly cross-inked three-dimensional solid network resulting in a gel. The composition, pore and primary particles sizes, gel time, surface areas, and density may be tailored and controlled by the solution chemistry. The gels are then subjected to either supercritical extraction or controlled slow evaporation of the liquid phase from the gel. Supercritical extraction of these gels allows the surface tension of the leaving liquid phase to be reduced to near zero and results in a highly porous skeletal structure which is low density aerogel, while controlled slow evaporation of the liquid phase from the gels leads to a xerogel, which may be high density. Applying stress during the extraction phase can result in high density materials. By use of the sol-gel processing, energetic materials can be made, for example, by solution addition solution exchange, powder/particle addition, functionalized solid network, functionalized explosive network, and micron to submicron scale composite energetic materials.
The methodologies of the above six sol-gel manufacturing techniques are briefly described as follows:
Solution Addition: The energetic material constituent is dissolved in a solvent which is compatible with the reactive monomer and mixed into the pre-gel solution prior to gelation. Upon gelation, the energetic material constituent is uniformly distributed within the pores of the solid network formed by the polymerization of the reactive monomer.
Solution Exchange: After gelation, the liquid phase is exchanged with another liquid which contains an energetic material constituent, thus allowing deposition of the energetic material constituent within the gel.
Powder/Particle Addition: The energetic material constituent, in particulate form, is either mixed with the pre-gel solution or added to a pre-made gel, resulting in a composite of gel and suspended particles.
Functionalized Solid Network: Use of reactive monomers which have functionalized sites dangling throughout the solid network after gelation. Dissolution of the energetic material constituent in mutually compatible solvents and diffusing into the gel allows the energetic material constituent to react and bind to the functionalized site. Thus, the amount of energetic material constituent may be controlled by the number of functionalized sites while ensuring homogeneity at the molecular level.
Functionalized Energetic Network: Functionalizing the energetic material constituent molecules so that they can be reacted in solution to directly form a three dimensional solid (gel) which incorporates the energetic molecules at the finest scale. In this embodiment, the solid network is the energetic material and, if desired, the concentration can be controlled by co-reacting with other inert reactive monomers.
Micron To Sub-Micron (Nano) Scale Composite Energetic Materials: Higher performance energetic materials can be made in which the skeletal structure and the surrounding phase serve as fuels and oxidizers to form a composite energetic material. Conductive gels (e.g., carbon aerogels) may be used as substrates for the electrochemical precipitation of metal fuels, or metals may be deposited within non-conductive aerogels via decomposition from the gas or liquid phase. Void space may then be used for the addition of an oxidizer and other energetic material constituents.
The sol-gel manufacturing of energetic materials solves many of the prior above-described problems associated with the manufacture of energetic materials.